Conventionally, when shifting gears, many powershift transmissions use solenoid controlled valves to control pressure to each clutch and rely on a signal that is representative of engine load to determine the pressure applied to the on-coming clutches.
A problem that may occur is that the engine load signal may be misleading. For example, in agricultural tractor applications, there are conditions where much of the engine load may be used to power auxiliary functions such as a hydraulic pump or PTO implements. This may cause problems where the shift quality is harsh because the oncoming clutch pressure is commanded at high pressure when instead it should have been commanded low because there was actually only a small amount of the engine power that was going to the drive wheels.
A method of overcoming this problem is described in U.S. Pat. No. 6,022,292. In this reference the load signal from the engine is adjusted to assume that part of the load is going to any auxiliary function that is engaged. The load signal is continuously adjusted automatically based upon the resulting shift characteristics. If the tractor slows down excessively during a shift, it is an indication that the assumption of engine load that is going to the wheels is too low. If the tractor speeds up during a shift, it is an indication that the assumption of engine load that is going to the wheels is too high. The problem with this method is that it can only react to a bad shift. It does not prevent the bad shift from occurring in the first place.
Another problem is that vehicle drivetrains must be designed to handle maximum engine power. However, in some cases some of the engine power is going to auxiliary functions such as PTO implements or hydraulic pumps. In, these cases, it may be desireable to increase engine power because the limiting factor (the drivetrain) is not being loaded to it's capabilities.
A method of determining the power going to the PTO implement is described in U.S. Pat. No. 6,729,459. In this method the PTO clutch pressure is brought down and maintained at a pressure that produces a constant small amount of slip in the clutch. By knowing the commanded pressure that caused slip, the amount of power going through the PTO clutch can be calculated. Besides providing torque measurment, this method also provides a method of protecting the PTO driveline from shock loading. This primary disadvantage of this system is the power loss that is inherent with continuously slipping the clutch.
Accordingly, there is a clear need in the art for a method of determining the amount of engine load going to an auxiliary function that avoids the foregoing problems.